1. Field of the Invention
This invention deals with multiple quantum well photodetectors intended for unpolarized radiation detection.
2. Description of the Related Art
One-dimensional multiple quantum well (MQW) photodetectors have been developed that are sensitive to a greater range of incident radiation than are intrinsic or extrinsic semiconductor photodetectors. With the MQW device, the operating characteristics are controlled by the width and height of the quantum wells, rather than by selecting from limited available materials as with semiconductor photodetectors. An MQW detector can be very thin, on the order of a micron or less, making it much more radiation hard than semiconductor detectors.
MQW detectors are formed from superlattice stacks of ultrathin semiconductor layers, typically Group III-V semiconductors. With these materials the energy bandgap is direct, permitting light to be efficiently emitted or absorbed without the aid of lattice vibrations. Input photons transfer energy to electrons in the wells, exciting the electrons from a ground state, while an electric field moves the electrons through the superlattice. The MQW materials are characterized by large charge carrier mobilities, and are easily doped with impurities. They can form solid solutions of various proportions with identical crystal structures and well-matched lattice parameters, but with different energy bandgaps and indices of refraction.
The detection of long wavelength infrared radiation (LWIR) with MQW detectors has been reported in several publications: Levine et al., "Bound-to-Extended State Absorption GaAs Superlattice Transport Infrared Detectors", J. Applied Physics Letters Vol. 64, No. 3, 1 Aug. 1988, pages 1591-1593; Levine et al., "Broadband 8-12 .mu.m High-Sensitivity GaAs Quantum Well Infrared Photodetector", Applied Physics Letters Vol. 54, No. 26, 26 Jun. 1989, pages 2704-2706; Hasnain et al., "GaAs/AlGaAs Multiquantum Well Infrared Detector Arrays Using Etched Gratings", Applied Physics Letters Vol. 54, No. 25, 19 Jun. 1989, pages 2515-2517; Levine et al., "High-Detectivity D*=1.0.times.10.sup.10 cm .sqroot.Hz/W GaAs/AlGaAs Multiquantum Well .lambda.=8.3 .mu.m Infrared Detector", Applied Physics Letters, Vol. 53, No. 4, 25 Jul. 1988, pages 296-298.
The described detector consists of a periodic heterostructure of gallium arsenide (GaAs) quantum wells and aluminum gallium arsenide (AlGaAs) barrier layers. The GaAs quantum well layers are doped with an n-type dopant such as silicon to provide electrons in the ground states of the wells for intersubband detection. Under the influence of a bias voltage applied across the MQW structure, electrons excited by incident radiation into a conduction band above the barrier heights flow through the device. This majority current flow is sensed and provides an indication of the amount of LWIR that is incident upon the device.
While one-dimensional MQW photodetectors made of heterojunction material provide flexibility in the optimization of performance for LWIR, especially in the greater than ten micron wavelength range, the quantum efficiency of these devices is limited for unpolarized light detection. This is because they are not sensitive to optical polarization parallel to the detector plane. To excite an electron in a well, the electric field associated with the photons must be perpendicular to the vertical barrier walls (in the first order).
Ideally, the light to be detected would be directed onto the detector at 90 to the detector plane, to obtain the best image. However, since the plane of polarization for a light beam is normal to its direction of propagation, this would place the polarization plane parallel to the detector, so that the beam could be not detected. To compensate for this, the light is normally directed onto the detector at an angle to the detector plane such that a substantial component of the polarization is perpendicular to the detector plane and thus absorbed; at the same time the angle is kept great enough to preserve adequate image clarity. In practice, an incident angle of about 45.degree. has been found to be satisfactory.
Since the component of the incoming beam that has a polarization parallel to the detector plane will not be absorbed by the MQW detector, an inefficiency is built into the system. As the angle between the beam and the detector plane increases, a thicker detector is necessary to obtain the same level of absorption. Thick detectors, however, are undesirable because they are less radiation hard and require a higher operating bias voltage.
The MQW detection problem for unpolarized light is illustrated in FIG. 1, in which an optical beam 2 is shown incident upon an MQW superlattice 4 at a right angle. (The term "optical" is used in its broad sense as including infrared and other regions of the electromagnetic spectrum which an MQW may detect, and is not limited to visible light.) The beam's plane of polarization, illustrated by arrow 6, is perpendicular to the beam, and thus parallel to the plane of the superlattice 4. Since all the photons are polarized in the plane of the superlattice, the beam will not be absorbed in the superlattice and will instead be transmitted through undetected.
The prior technique of directing the beam at an acute incident angle to the MQW superlattice is represented in FIG. 2. The photons of an unpolarized optical beam 8 can be represented by a pair of orthogonal polarization vectors in an arbitrary direction normal to the direction of propagation. For a beam with a 45. incident angle, the polarization vectors can be represented by vector arrows 10 and 12, which lie respectively in planes parallel and perpendicular to the superlattice 4. The photons that are polarized perpendicular to the superlattice in plane 12 will be absorbed as the beam progresses through the superlattice (assuming the superlattice is sufficiently thick), while the photons polarized in plane 10 parallel to the superlattice will not interact with the one-dimensional MQW detector. The output beam after transmission through the superlattice will thus be linearly polarized along a vector 10' parallel to the superlattice. Since this component of the beam has gone wholly undetected, the maximum achievable quantum efficiency of the described MQW photodetector is 50%.
In an attempt to resolve this problem, are fraction grating that reflects and alters the angle of an incident beam of radiation has been placed on the opposite side of the MQW detector from the incident beam. This approach, illustrated in FIG. 3, is described in U.S. Pat. No. 5,026,148, "High Efficiency Multiple Quantum Well Structure and Operating Method", issued Jun. 25, 1991 to Wen et al. and assigned to Hughes Aircraft Company, the assignee of the present invention.
According to this patent, an incident ray 14 directed through a transparent substrate 15 onto an MQW photodetector 16 at an off-normal angle is partially absorbed as it propagates through the detector. Unabsorbed radiation with a polarization parallel to the superlattice is reflected back through the superlattice, at an off-normal angle to the detector surface, by a reflection grating 18 on the rear of the detector. The polarization of the reflected ray is thus shifted to a substantially non-parallel angle to the superlattice. This allows the superlattice to absorb at least part of the reflected radiation during its second pass through the detector. The reflection grating is implemented as either a reflective saw-tooth surface, or a periodic reflective grating pattern.
The patent also describes a version of the photodetector that is designed for incident light normal to the superlattice. This variation is illustrated in FIG. 4. An angular shifting mechanism such as an optical transmission grating 20 is formed on the forward surface of the transparent substrate 15', and redirects a normal input beam 22 into a pair of off-normal subbeams 22a, 22b within the detector. The subbeams are partially absorbed in the superlattice, with the unabsorbed portions reflected off a reflection grating 18' at the rear of the detector. The reflection grating 18' is oriented at a cross-angle to the transmission grating 20. It thus reflects the subbeams 22a and 22b at new angles at which their polarization is again at least partially normal to the superlattice, so that the reflected subbeams are absorbed during their second pass through the detector.
While the cross-grating structure of FIG. 4 provides for up to 100% absorption of incident radiation that reaches the detector in a direction normal to the MQW superlattice, the transmission grating 20 must be located very close to the detector. Otherwise the diffracted light will undergo an excessive lateral spreading that results in pixel "smearing" and serious loss of resolution. The laterally diffracted light consists of both a pair of first order lobes, and higher order diffracted side lobes. With an array of detectors, this lateral spread will result in poor isolation among different detector elements. In a practical device it is difficult to fabricate the transmission grating sufficiently close to the MQW superlattice. With current pixel sizes down to about 40.times.40 microns, the MQW substrate 15' would have to be comparably thin to avoid pixel smearing. This would result in a very fragile structure.
Another approach to obtaining a non-normal incident angle from radiation that is originally normal to the detector plane involves forming an edge of the substrate upon which the detectors are grown at a non-normal angle, typically 45.degree.. This is discussed, for example, in Goossen et al., "Photovoltaic Quantum Well Infrared Detector", Applied Physics Letters, Vol. 52, No. 20, 16 May 1988, pages 1701-1703. However, since only that portion of the incident radiation that falls upon the angled substrate edge will be redirected at a non-normal angle to the detector, the radiation that falls upon the remainder of the substrate will not contribute to a detector output. This approach can be used for a single detector or for a small detector array, but is not useful for larger arrays. Also, the process of forming an angle in the substrate edge places the detectors at risk. Larger detectors arrays can be accommodated by mounting the detector substrate upon a second and considerably larger substrate, with the angled edge moved to the second substrate. Since the second substrate is considerably thicker than the first substrate, it can be configured so that the area of its angled edge is sufficient to illuminate the detector array with non-normal radiation in response to normal radiation incident upon the angled edge. However, the detector structure is relatively fragile because the superlattice chip is not solidly supported in a normal mounting configuration. The weakness in mechanical structure can result in low yield and poor sensor reliability, especially since bandwires must be attached from the detectors to the readout circuitry.